Carburetor Heat Requirements

Although the Ellison Throttle Body Injector is much less prone to carburetor ice than
conventional float type carburetors, under certain conditions it can accumulate ice and a
carburetor heat system is therefore required. To understand why carburetor heat is a
requirement it is helpful to understand where and how carburetor ice forms in both a
typical float-type carburetor and in the Throttle Body Injector:

Float Type Carburetors

1. The throttle plate experiences drastic cooling due to the evaporation of fuel which
is sprayed on it by the upstream fuel jet. The thermodynamic "venturi" effect,
which is popularly assumed to be responsible for ice formation, is insignificant. Water
vapor in the air condenses on the cold throttle plate and freezes, with ice accumulating
on both sides. Engine roughness from distortions in cylinder to cylinder fuel distribution
is often the first warning symptom, followed by substantial power loss as the growing ice
deposit obstructs induction airflow. Final engine failure can either be a rich or lean
failure depending on subtleties of ice ball geometry.

2. Another form of icing can occur in Marvel-Schebler carburetors (as well as in the
Ellison TBI as explained below) when small air bleed passages in the idle system as well
as the main metering circuit are occluded with one or more small particles of ice. The
result is excessive rich roughness that may occur over a narrow RPM range or may be
present over the full range of throttle openings. When the engine finally quits it is a
rich failure.

Throttle Body Injectors

The Ellison Throttle Body Injector (TBI), with no throttle plate downstream of the
point of fuel injection, at first glance appears to be ice proof. Unfortunately, many of
our customers have never gone beyond the first glance in spite of our warnings. A TBI, in
any given application, may go years without accumulating any ice. Unfortunately, some of
our customers feel that since the TBI ices up so infrequently, they won't be quite as dead
as if it happened often. As with the Marvel-Schebler carburetor, the same two ice
mechanisms are possible in the TBI. Here's how:

1. The same extreme temperature drop occurs in the TBI as with the M-S and for the same
reason, namely fuel evaporation. With no throttle plate downstream of the fuel input there
is little for the ice to adhere to. It is possible however, for ice to form on the
sidewall of the TBI downstream of the throttle slide. TBI owners can convince themselves
of this danger by running their engine at 1500 RPM for a while on a humid day with the
cowl removed; the diffusion section of their TBI will look like a Popsicle due to
atmospheric moisture condensing and freezing on the outside of the TBI. Deadly ice
formation can also occur in the manifold of engines with long cold intake pipes such as VW
conversions and Continental engines, to name a few. Updraft cooling can exacerbate
the potential for manifold ice because there is no warm air from the engine cylinders
flowing down over the induction tubes.

2. Idle circuits of most of our TBI models include an air bleed passage that makes the
fuel behave as though it had much lower viscosity. On humid days, when operating at idle
for extended periods of time, the bleed orifice (located in the bottom of the slide
cavity) will ice over causing idle mixture to slowly enrich. Cycling the throttle a couple
of times will sometimes clear the ice but the application of carb heat is the only sure
cure. As the fuel contribution of the idle circuit diminishes as the throttle is opened,
bleed circuit icing has negligible effect on power but can result in the engine quitting
in a decent if the throttle is retarded far enough. This is a critical problem in Eze's
because the driver, not being able to see the prop, has no good way of knowing that he's a
glider pilot.

Summary

For certificated aircraft, FAR 23.1093 requires that the intake air heat system must
provide a heat rise of 90°F at 75% power. It makes a great deal of sense to comply
with this regulation when building an "Experimental" category aircraft. To
prevent engine failure caused by carburetor or induction ice, this type of heat system
must be installed on all aircraft (including experimental) which use the Ellison Throttle
Body Injector.